Who can explain the relationship between prop pitch and diameter?
I know that for electrics, most guys prop relative to achieving the performance they want with at least some measure of efficiency. But relative to flat speed and climb, are there rules of thumb for pitch & diameter?When is it better to increase or decrease them?

Well, since no one else is chiming in, I will do my best. The best way I can explain it is to make an analogy to a transmission. Low pitch numbers = a low gear which means more torque. A high pitch number = a higher gear which means more speed. The pitch number is the number of inches the prop would move if turning through a solid (much like a metal screw through wood). Generally speaking, fast hotliner type planes have near equal diameters and pitch sizes such as a 5x5 or 6x6 and 3D type aircraft are more likely to have large diameters and lower pitch values such as 11x4.

If the pitch is too low, it is possible that even at full throttle, you won't generate enough airspeed for the wings to produce sufficient lift. It is also possible to over pitch to the extent that you waste energy. For example if you are spinning the prop so fast that it would be going say 90mph through a solid, and your plane is only going 50mph, then the prop is not very efficient for this application as it is just stirring the air without producing much thrust.

As far as the Corsair v. P-51 question, generally speaking larger prop diamter is more efficient then adding blades. In fact, a single bladed prop is very efficient! The corsair had gull wings so it could spin a larger diameter prop, the P-51 didn't have as much ground clearance and so the only choice was to add another blade.

To add more confusion, there is also the airfoil of the prop and the prop width to contend with. What I have said here are generalities that are open to debate or criticism. There is much info on the web on this subject, just do some searches and you can get as technical as you care to.

Hope this helps. There are smarter people on this board who could propbably elaborate but I saw no one was responding to your post.

More blades is equivelent to a larger diameter at the same pitch, with somewhat less than optimum efficiency. However when you aircraft has limited ground clearance, a lot of power, and the blade tips are already approaching the speed of sound, sometimes more blades is the only way to go.

I suspect that if you gear down enough as well, the effciency loss is not so marked. And there may well be striuctural reasons (stronger blades etc) for using more small ones rather than two big ones.

It has been said that on our very small airplanes, because our propellors turn at such a high speed, it is possible for each propellor blade to be running, at least partially in the wake turbulence of the blade ahead of it, and that this efficiency robbing tendency is exacerbated by three or four blade props.

I have seen lengthy discussions accompanied by mathmatical formulae and drawings that purport to prove this to be true. But conversely, I have seen equally convincing efforts to prove the opposite.

At one time I had an opinion one way or the other, but I don'r remember what it was...

Tracon...I got virtually the same exact #s for the 8x720 NiMh pack: 59% for 2 blades and 56% for 3...MotoCalc doesn't have the 730s yet but I assume their performance is virtually identical to the 720s.

If you haven't gotten MotoCalc, I sure do recommend it (this coming from a technical nincompoop who aspires to one day sound credible...at my funeral I suppose!)

I posted this message because any bozo can plug numbers into a calculator; but I want to understand WHY 3 props are seldom better than 2, how pitch and diameter are related, etc.

How much difference is there going form a 5x2.5 up to a 5x3? This is on a wattage hawk ep with a 400 size directdrive. the 2.5 broke in a kitchen accident and all the hobby shop has is the 5x3. Is that 1/2 inch in pitch enough to even notice?

Going from 2.5 to 3 is a 20% pitch increase, creating about 15% more pitch speed at the expense of a slightly higher current draw. You may see a small increase in maximum speed, and slightly better climb rate.

Quote:

Tracon:
So what is more effective on a GWS 350 motor on a C gearing a 9X7 2 blade or a 9X7 3 blade ???

According to my tests there is very little difference between them. The 3 blade prop seems to produce slightly less thrust, whilst consuming slightly more power. The only reason I can think of to use it is for appearance!

Don Stackhouse there designs full size propellers as a day job (the runs DJ Aerotech at other times selling the roadkill series of models). What he doesn't know about propeller design simply isn't worth knowing!

....
According to my tests there is very little difference between them. The 3 blade prop seems to produce slightly less thrust, whilst consuming slightly more power. The only reason I can think of to use it is for appearance!

Really?! I would ever tought in a greater difference about power consumption.
It seems that the efficiency of a 3 blade propeller is very low!
I would have choose a smaller diameter 3 blade prop to match a 2 blade one with the same pitch, to keep all other specs the same.
Could the results of your test confirm the theory of turbolence in a scale propeller? It seems that the third blade is almost transparent.
Anyway I agree with you about the use of a 3blade prop: just for a scale plane!
Piero.